Denti,
The 737 FCOM i have available (20.05.2010) states
I should have stated that I am flying the Classic and that I'm referring to the FCTM of 2008. Ours states 25+15 for full manoeuvring capability.
Anyway, the key point as far as I am concerned is that when you retract flaps on take off using the normal procedure, you will be below the flap manoeuvring speed for the newly selected flap. Whether this is considered unsafe or not is another debate. But that fact is that you are below that speed and as such margins are reduced. This I suspect is the single key issue that must be considered when retracting flaps in a turn after takeoff which is why I associated the immediate turn procedure with this thread.
As I previously stated some of the advice in our FCTM is contradictory and it sounds like this has been cleared up in later versions. Therefore some of this is open to interpretation, but "overshoot" is circumstantially defined as "inadvertent overshoot" (to quote our FCTM). Therefore, unless there is specific guidance that applies the situation in question (such as the written Immediate Turn After Takeoff procedure) and the fact VNAV will command 30º, my view is that 25+15 means that I'll limit my bank angle to 25 (plus minus a degree of human error) and allow 15 for such an inadvertent overshoot. That doesn't mean anything else would be officially or otherwise unsafe, but after gathering all the information and working on a risk/benefit basis that is my choice until our manuals are updated.